gas: add README

plugins/gas/README.md

@@ -0,0 +1,9 @@
+# Gas plugin
+
+This plugin adds autocompletion for the [gas](http://walle.github.com/gas) command,
+a utility to manage Git authors.
+
+To use it, add `gas` to the plugins array of your zshrc file:
+```zsh
+plugins=(... gas)
+```